This invention relates to a heating, ventilating and air conditioning (IVAC) system wherein several interconnected units communicate control signals to and from each other over a communication bus.
HVAC systems are becoming increasingly complicated. As these systems increase in complexity, and sophistication of control, the number of wires that are hard-wired between the system components and controls increase. As an example, traditional residential HVAC systems have a thermostat on the wall of the home that allows a user to set a desired operating mode and temperature. There is also typically an indoor unit (a gas furnace or a heater/fan) and an outdoor unit (air conditioner or heat pump). Even this simple system illustrates the problem mentioned above.
HVAC systems operate on a simple control protocol. Based upon user-desired settings at the thermostat, and actual room temperature, command signals are sent to the indoor and/or outdoor equipment to perform heating, cooling, or fan functions. In the majority of the present systems, each of these functions requires a dedicated wire to run between the thermostat and the respective piece of equipment. The thermostat switches on a 24 volt AC signal over the wires to command the equipment to turn on a desired function, and removes the 24 volt AC signal to turn off the function.
Some systems have extended this protocol to include an additional wire for carrying fault information from one piece of equipment back to the thermostat, and displaying fault information to the user of the HVAC system. As residential systems become more sophisticated, multiple stages of heating and cooling are becoming common. Here again, the traditional protocol has been extended to include an additional wire for each equipment stage. New functions, such as controlling humidity, are also being integrated into thermostats. Again, each function has typically required its own wire.
As an example, one thermostat currently manufactured by the assignee of this application includes the ability to connect up to 11 wires. Two of the wires provide the 24 volt power, and two of the wires extend to an outdoor air temperature sensor. Seven other wires control various functions at the indoor and outdoor HVAC units. As an example, separate wires are required to turn on the indoor unit and to move it between various speeds or stages. These 11 wires do not include any of the “feedback” or status information as mentioned above. To provide this feedback would require even more wires. As can be appreciated, this results in a very complex installation, as each of the 11 wires must be attached at the correct location on the thermostat. Each of the seven control wires provide a single control function. As the number of wires grows, so does the installation complexity and possibility of mis-wiring. Compounding this problem, each combination of equipment (fan or furnace, AC or heat pump, one-stage or multi-stage, humidifier or not, etc.) has a different wiring arrangement. All of this can be challenging for a less experienced residential HVAC installer. This can lead to poor installation, resulting in degraded performance, malfunction, or service calls.
In many existing homes, the above challenges are complicated in that there are only four wires run through the wall to the thermostat at set-up. It may be difficult or impossible to run extra wires to upgrade functionality.
Some systems have included somewhat more sophisticated controls. As an example, the assignee of the present application developed a thermostat control which communicates multiple control signals over two control wires to a main control panel. However, the main control panel is still hard-wired to the indoor and/or outdoor units. As such, there is still the problem mentioned above with regard to an undue number of wires.
Thus, a simplified system would be desirable that addresses the above-referenced problems and allows for easy system upgrade.